A method and an apparatus for monitoring a catalytic converter are known from U.S. Pat. No. 5,182,907. A sensor is arranged in front of and behind the catalytic converter, in each instance, to detect the proportion of hydrocarbons in the exhaust gas. A defect of the catalytic converter is recognized on the basis of a comparison between the hydrocarbon content in front of and behind the catalytic converter. A disadvantage here is that two sensors are required.
The exhaust gases of self-ignition internal combustion engines tend to produce high NOx emissions, because of the fact that they are operated with a great excess of air. This occurs to an even greater degree in internal combustion engines with direct fuel injection into the combustion space. A reduction catalytic converter can be used to reduce these emissions. To reduce the NOx levels, additional fuel is metered in ahead of the catalytic converter, and this fuel is converted in the catalytic converter.
Furthermore, it is provided that in the case of diesel engines, the exhaust gas is subsequently treated by an oxidation catalytic converter. This makes it possible to clearly lower the hydrocarbon emissions (HC emissions). Such a method of procedure is described, for example, in German Patent No. 4441261.
In the United States, so-called On-Board-Diagnosis (OBD) is required. This means that certain components are constantly monitored, and defects, particularly a deterioration in the exhaust gas emission values of the engine, are reported to the driver. For the NOx catalytic converter, monitoring is required because of the amount of fuel that is metered in.